Blender random packing animation (reactor cylinder type catalyst random packing)
Make your Steps Per Second high enough. This is in Scene tab under Rigid Body World and Settings. Default is 60 and seems to be too low in some cases. High value will slow down calculations but will be more accurate.

Next make sure your collision Shape is Mesh for you funnel. This is important for complex objects. For your pellets I think Capsule should be fine. Also make Sensitivity Margin small enough. Accurate shape will slow down calculations but will be more realistic.

Last make sure your walls are thick enough. If your wall are too thin then your particles can make through before next step happens.